1. Field of the Invention
The invention relates to a method for the interferometric measurement of non-rotationally symmetric wavefront errors on a specimen brought into a number of rotational positions, at least one measurement result being determined in each of the rotational positions, and a concluding mathematical evaluation of all measurement results being performed.
2. Description of the Related Art
A fundamental requirement of each precision measurement technique consists in determining the measurement instrument error. Provided for this purpose are standards whose error contributions are either known or can be separated from the measuring instrument error by suitable measuring methods. The error of the measuring instrument is determined from the respective measurement and the error of the respective standard. In many cases, however, no such standards are available, for which reason it is necessary to depend on methods for separating the contributions of measuring instrument errors and testing surface.
In the optical surface measuring technique, testing surfaces are measured with the aid of an interferometric measuring method. For this purpose, a shape-matched wavefront is aligned with the specimen, and the shape of the reflected wavefront is measured. In addition to the surface shape of the testing surface, errors of the measuring instrument are also found impressed onto this wavefront.
As regards the general prior art, reference is made to the publications by R. Freimann, B. Dörband, F. Höller: “Absolute measurement of Non-Comatic Aspheric Surface Errors”, Optics Communication, 161, 106-114, 1999; and C. J. Evans, R. N. Kestner: “Test Optics Error Removal”, Applied Optics, Vol. 35, 7, 1996; the JP 8-233552 and the U.S. Pat. No. 5,982,490.
The possibility in principle of using two independent measurement series is pointed out specifically on page 1018, 2nd paragraph, in the publication by Evans and Kestner listed above, although no specific method is named. Moreover, it is pointed out explicitly at this juncture that such a method requires specific assumptions on the errors to be expected, and a (mathematically complicated) fitting of the measured data.